The present invention relates generally to an apparatus and method for detecting workpiece defects and, more particularly, to a dark field inspection apparatus and method for detecting defects on a workpiece in a clean room environment.
Workpieces such as, for example, wafers, are typically in the form of a flat, substantially planar disk. Workpieces may include semiconductor wafers, magnetic disks and optical disks. For many applications, particularly in the area of integrated circuits, the wafer serves as a high-tech building block. In order to produce quality microelectronic devices, it is desirable that the wafer surface be uniform, planar and devoid of any imperfections.
Chemical mechanical planarization (CMP) is an abrasive process used for polishing the surface of the wafer. The CMP process involves the use of chemical slurries and a circular (sanding) action to polish the surface of the wafer. Slurry is a chemical polishing agent deposed upon the wafer while on a polishing pad. After the polishing step, the wafer is cleaned and rinsed to remove the slurry. A wafer may undergo several steps of cleaning, rinsing, polishing and drying in the CMP machine to remove any debris from the wafer surface. For example, the manufacture of a semiconductor wafer generally includes xe2x80x9clayeringxe2x80x9d dielectrics or metals on the wafer surface. After each layer is formed, the wafer surface is typically cleaned, polished, rinsed and dried. The smooth surface is now ready for further processing steps or for the next layer to be applied to the surface.
The CMP process to some degree smooths out minor defects in the wafer surface which result from, for example, slicing the wafer from a silicon ingot. However, the CMP process itself can introduce or reveal several types of yield-limiting defects, including residual slurry, surface voids and surface particles. Residual slurry remaining on the wafer as a result of inadequate cleaning can unevenly raise the surface level.
Wafer surface defects are considered xe2x80x9ceventsxe2x80x9d that lead to or can lead to electrical faults in microelectronic circuitry. The events are first identified and then reviewed to determine if they represent defects that can adversely affect subsequent device performance. Defects are found on both patterned and unpatterned wafers. An unpatterned wafer is a bare silicon wafer or a wafer with various blanket films coated thereon. A patterned wafer has undergone a photolithography process wherein geometric shapes have been transferred to the surface of the wafer.
A surface void is a divot on the wafer surface caused by an embedded particle (e.g., dust) or a weak point in the top layer that is ripped out or dislodged during processing. For example, a small foreign particle may be xe2x80x9ccoatedxe2x80x9d on the surface during the layering step and subsequently dislodged from the surface. Thus, a void or dimple remains where the particle once was. Alternatively, surface particles may adhere to the surface after the layering step resulting in a small rise (mound) in the surface. Similarly, the dielectric or metal used in the layering step may become contaminated with a trapped particle prior to or during the layering step. Once applied, the layer will exhibit xe2x80x9croughxe2x80x9d areas where contaminates are present.
Microscratches on the wafer surface are caused by small particles, debris or similar foreign objects caught between the polishing pad and the wafer surface. During the polishing step, the particle causes a xe2x80x9cscratchxe2x80x9d on the wafer surface. In some cases, the microscratch may be inadvertently filled with a deposited material (e.g., tungsten) during a tungsten CMP process application.
After the wafer has undergone a CMP process, including layering the wafer with dielectrics or metals, the wafer proceeds to a photolithography process. Photolithography is a means for transferring shapes from a mask to the surface of a wafer, producing a patterned wafer. The steps involved in the photolithographic process will be briefly discussed. First, the wafer is chemically cleaned to remove any remaining foreign matter from the surface such as, for example, traces of organic, ionic and metallic impurities.
Silicon dioxide (SiO2) maybe deposited on the surface to serve as a barrier layer, whereupon a photoresist maybe applied to the surface. The wafer is spun at a high speed, called xe2x80x9cspin coating,xe2x80x9d to produce a thin uniform layer of photoresist on the surface. Traditionally, there have been two types of photoresist, positive and negative. Negative resists were common in the earlier years of integrated circuit processing, but more recently positive resists are the prevalent type of resist used in VLSI (very-large-scale-integration) fabrication processes. For positive resists, the wafer is exposed with UV (ultraviolet) light wherever the underlying material is to be removed. In this resist, exposure to the UV light changes the chemical structure of the resist so that it becomes more soluble in the developer. The exposed resist is then washed away by the developer solution, leaving windows of the bare underlying material. The mask used contains an exact copy of the pattern which is to remain on the wafer.
The photoresist coating becomes photosensitive, or imagable, after a soft-baking step. During soft-baking, nearly all of the solvents are removed from the photoresist coating. If considerable solvent remains in the coating, the positive resist may be incompletely exposed.
The mask is a glass plate with a patterned emulsion of metal film on one side. The mask must be aligned with the wafer in the location that the pattern is to be transferred to the surface. The glass plate is a lens which directs the UV light onto the wafer. Exposure time can vary depending upon, for example, the sensitivity of the resist and the lens aperture. Once aligned, the photoresist is exposed through the pattern on the mask with a high intensity UV light, creating on the wafer surface a xe2x80x9cprintedxe2x80x9d replica of the mask pattern.
The wafer is then placed in a developer solution until the resist becomes completely soluble (i.e., for positive resists), and then hard-baked to improve adhesion of the photoresist to the wafer surface and to cure the photoresist.
While not as common as in the CMP process, defects can and do occur during the photolithographic process. Briefly, the defects caused by photolithography can be grouped into three areas; (1) defects in the resist material, (2) problems with the pattern or equipment, and (3) errors in the printing or exposing processes. The photoresist material may become contaminated with a foreign particle, material or substance prior to spin-coating. As a result, the photoresist will contain small spurious particles causing an event. Additionally, residual resist (resist remaining after the developer step) creates xe2x80x9cwebbingxe2x80x9d between the printed lines on the wafer during the exposure step.
A printing defect can result from anomalies on the pattern. Any damage to the pattern (e.g., scratches) can affect the wafer duplication. Lastly, printing errors can cause defects such as, for example, a xe2x80x9cbridgingxe2x80x9d between two printed lines and large areas of unexposed photoresist.
Wafer manufactures ambitiously attempt to prevent defects due to the substantial threat of reduction in final wafer yields. Typically wafer production takes place in a clean room environment. A clean room is broadly defined as an uncontaminated or nearly uncontaminated room which is maintained for the manufacture or assembly of objects, e.g., semiconductor wafers. U.S. Federal standard 209E catagorizes clean rooms into classes defined by the levels of air cleanliness expressed in number of particles per cubic measure. For example, a class 100 room must have less than 100 particles per cubic foot; a class 10 room must have less than 10 particles per cubic foot, and so on. In general, as the class room number decreases, the number of particles per cubic measure allowed decreases.
Many of the particles present in the air, on clothing and on skin, if brought in to contact with a workpiece, can fatally contaminate the workpiece (i.e., workpiece must be discarded). Contaminates may include dust particles, skin, hair, body oils, fibers, and even small living organisms such as dust mites. Potentially, each of these contaminates can fall on a workpiece or in a material used to xe2x80x9ccoatxe2x80x9d the workpiece and subsequently result in an event.
Workpiece (e.g., wafer) defects can be reduced by limiting contaminates in the manufacturing room to less than 10 particles per cubic foot (class 10). To achieve such a low rate of xe2x80x9cloosexe2x80x9d particles in the air, on the equipment and on the human operators, clean room technologies are employed. Continuous air flow, ventilation and filtration are essential to maintaining low particle count. Hepa filters may be installed in the ceiling and floor to continuously draw air in for particle filtering. The floors may be raised in a grid-like fashion to allow air venting under foot. The operators can comfortably walk atop the grids without noticing the spacing between the grids which allows air and large contaminates to flow through the raised floor. This reduces the risk of operators xe2x80x9cpicking upxe2x80x9d fallen particles while working in the clean room.
Perhaps the most common source of contaminates and the hardest to control in a clean room environment are human contaminates. Skin fragments and hair follicles are continually shed from the human body. Each time a workpiece is handled by a human operator, skin, hair and body oils may be inadvertently transferred from the human to the workpiece surface. To prevent this, operators are typically required to wear full body suits made from lint-free material. Nearly every inch of human skin is covered as well as the operator""s shoes. The head is hooded with several layers of material to keep hair from dropping in the room. The nose and mouth must also be covered to help prevent particles from being cast into the air during exhale. Again, clean room ventilation plays an important part in filtering the air to collect exhaled particles.
Even with the precautions mentioned above, wafer defects nonetheless occur during manufacture (e.g., CMP and photolithography). Wafer defect detection, analysis, and resolution play an important role in keeping wafer processes and yields under control. In fact, as long as there has been semiconductor manufacturing, and more particularly VLSI circuitry, defect inspection has been vital to identifying sources of potential electrical faults. As disclosed previously, the types of defects can vary depending upon which production stage the defect originates (e.g., pre-CMP, post-CMP, lithography). Many manufacturers must perform intermittent inspections throughout production searching for different types of defects each time.
Companies such as KLA-Tencor Corp. have introduced high tech tools for patterned and unpatterned wafer inspection. For example, KLA-Tencor""s AIT II is a patterned defect inspection tool that offers automated double-dark field (DDF) laser scanning technology, real-time signal processing algorithms, and an automatic defect classification for analysis. The Surfscan(copyright)SP1 inspection tool, designed by KLA-Tencor, performs highly sensitive inspection of unpatterned wafers. The SP1 features multiple dark field collection optics and an optional bright field channel and can detect and classify defects on all types of unpatterned surfaces. An advanced feature of the SP1 called Surface NanoTopography(copyright) (SNT) provides quantitative measurements of topography variations with nanometer sensitivity over millimeter ranges.
New process technologies such as CMP introduce unknown classes of surface defects requiring companies to continuously update and advance their latest defection tools. As wafer manufacturing processes increase in complexity, defect detection costs are driven up. For example, an automated wafer detection tool as previously described can cost the wafer manufacturer $500,000 or more, and with each new advanced feature further increasing the overall cost. Keeping up with defect detection may pose a daunting economic challenge even the largest semiconductor manufacturers.
Human inspectors may also be employed to inspect workpieces and can after spot defects as small as 0.5 xcexcm. Typically, not every wafer is inspected, but rather a statistical sampling of wafers is inspected from a particular cassette or lot. In practice, the operator removes a wafer from a cassette and inspects the wafer for surface defects. Because the CMP and photolithography processes are generally performed on a cassette or xe2x80x9clotxe2x80x9d of wafers at a time, a periodic sampling of the cassette should give an accurate account of any post-production defects present in the entire cassette. If the inspector discovers a defect on a sampled wafer, additional wafers may be inspected from the cassette. Upon discovery of a defect, the inspector may view the wafer under a microscope located at the inspector""s inspection station. Microscope viewing is also useful for xe2x80x9cspot-checkingxe2x80x9d the printed lines on the wafer and may be performed at any time during the inspection process.
As previously discussed, a photoresist pattern is duplicated onto the wafer surface. FIG. 1 illustrates an exemplary resist pattern atop an oxide layer which is coated on the wafer surface. Both the resist patterns 100 and the oxide layer 125 are light transparent, therefore light can pass through to the wafer surface 150 where it is reflected back. Light which is directed on-axis to the wafer 130 is high intensity and can be easily detected with the human eye. The reflected light 140 is substantially equal in intensity to the incoming light 130 and is equally easily detected by the human eye.
Light photons are scattered from the edge of the resist features 160. The scattered light intensity is much less than the intensity of the incoming 130 or reflected light 140 and, hence, the contrast is very low making it extremely difficult to detect the patterned resist. Defects in the pattern are even more difficult to spot over the high intensity incoming and reflected light. To avoid this problem, the high intensity light is blocked to create a xe2x80x9cdark field option.xe2x80x9d The scattered light from the edge of the resist features is more easily seen in a dark field option.
Presently known dark field methods used by inspectors are grossly inadequate and include simulating a dark field by physically holding a covering over the wafer. Simply, the inspector uses one hand to hold an opaque, such as black or similar non-transparent hue, piece of paper over the workpiece (e.g., wafer) and the other hand to hold the workpiece. Ideally, the paper will block the surrounding light thus creating a dark field to view the wafer. In practice, however, to effectively block the light from all angles, the paper has to completely encompass the wafer or at least encompass the sides, top and back. Holding a light shielding structure large enough to adequately surround the dimensions of the wafer (i.e., wafers average about 8 to 10 inches in diameter), while permitting the inspector to move the wafer to observe the surface, is cumbersome and awkward.
Current dark field techniques are not well suited to a proper clean room environment. Even with the extensive measures discussed above for a class 10 clean room, particles such as dust, clothing fibers and debris are still present near the inspector""s work station. Introduction and movement of objects at or near the station causes the particles to xe2x80x9cstirxe2x80x9d and become afloat in the air. Proper ventilation at the inspection station helps to draw loose particles from the air into the filter systems either above or below the station. Presently, each time the inspector readjusts the handheld dark field structure, particles are stirred-up and set afloat. The air between the structure and the wafer is filled with loose particles which can drop upon the wafer. The structure does not contain adequate ventilation to draw the particles away from the wafer because even the smallest hole in the structure would allow light to shine through and thus destroy the operator""s dark field.
Accordingly, there exists a need for a workpiece inspection method and apparatus that is cost effective yet reliable. More particularly, there is a need for a dark field inspection method and apparatus to assist human inspectors in detecting workpiece, such as for example, wafer, defects. Further, there is a need for a method and apparatus that enables wafer inspectors to undergo defect detection with a dark field option while maintaining the integrity of a clean room environment.
The present invention overcomes the problems outlined above and provides an improved method and apparatus for workpiece inspection. More particularly, the present invention provides a dark field option for workpiece inspection particularly useful in a clean room application. A preferred embodiment of the present invention comprises a structure having three sides and a multi-dimensional top. Air is allowed through the top surface of the structure.
In a preferred embodiment, a dark field inspection structure includes a top having a plurality of spaced-apart slats. The slats of each layer are offset from the slats of the next layer. The offset configuration substantially blocks the incident light by either reflecting or absorbing the light. A spacing defined by each slat and layer of slats permits air to flow through the top.
In one embodiment, the inspection structure comprises a substantially opaque and antistatic material.
In another embodiment, the inspection structure comprises a thin slot for transporting a workpiece.